Experimental and Theoretical Study of Electronic and Hyperfine Properties of Hydrogenated Anatase (TiO₂): Defect Interplay and Thermal Stability

Abstract

Performance of TiO2-based materials is highly dependent on the electronic structure and local defect configurations. Hence, a thorough understanding of defects interaction plays a key role. In this study we report on the results from emission 57 Fe Mössbauer Spectroscopy experiments, using dilute 57 Mn implantation into pristine (TiO2) and hydrogenated anatase held at temperatures between 300-700 K. Results of the electronic structure and local environment are complemented with ab-initio calculations. Upon implantation both Fe 2+ and Fe 3+ are observed in pristine anatase, where the latter demonstrates the spin-lattice relaxation. The spectra obtained for hydrogenated anatase show no Fe 3+ contribution, suggesting that hydrogen acts as a donor. Due to the low threshold, hydrogen diffuses out of the lattice. Thus showing a dynamic behaviour on the time scale of the 57 Fe 14.4 keV state. The surrounding oxygen vacancies favor the high-spin Fe 2+ state. The sample treated at room temperature shows two distinct processes of hydrogen motion. The motion commences with the interstitial hydrogen, followed by switching to the covalently bound state. Hydrogen out-diffusion is hindered by bulk defects, which could cause both processes to overlap. Supplementary UV-Vis and electrical conductivity measurements show an improved electrical conductivity and higher optical absorption after the hydrogenation. X-ray photoelectron spectroscopy at room temperature reveals that the sample hydrogenated at 573 K shows presence of both Ti 3+ and Ti 2+ states. This could imply that a significant amount of oxygen vacancies and -OH bonds are present in the samples. Theory suggests that in the anatase sample implanted with Mn(Fe), probes were located near equatorial vacancies as next-nearest-neighbours, whilst a metastable hydrogen configuration is responsible for the annealing behaviour. The obtained information provides a deep insight into elusive hydrogen defects and their thermal stability.

Local Environments in Iron-Bearing Clay Minerals by DFT Approaches: The Case of Structural Fe in Kaolinite

“…In this sense, it has been shown that dispersion corrections may play a significant role in the correct description of the structural parameters in phyllosilicates (Tunega et al, 2012) and, for Fe-bearing clay minerals, an accurate description of the 3d-electrons of Fe generally requires the use of Hubbard corrections (the so-called DFT+U method) (Geatches et al, 2013;Kéri et al, 2017Kéri et al, , 2020. On the other hand, DFT-based methods have been shown an overall good performance for predicting Mössbauer parameters in many different structures (Bochevarov et al, 2010;Pápai and Vankó, 2013;Zyabkin et al, 2020;Mudarra Navarro et al, 2021). Then, considering all this experimental and theoretical background, it becomes relevant to assess the DFT methods for predicting structural Fe local environments in kaolinite.…”

Titanium Oxide Electrocatalytic Membrane Filtration: “Two Faces” of Oxygen Vacancies in Generation and Transformation of Reactive Oxygen Species

“…The presence of Ti 3+ and Ti 2+ suggests the formation of oxygen vacancies in TiO x EMs. − Then, we first semiquantified the surface oxygen vacancy concentration through XPS spectra of O 1s (Figure e). The O 1s peaks centered at binding energies of 530.6–530.9 eV are attributed to the lattice oxygen atoms, while those at binding energies of 532.1–532.7 eV are ascribed to the adsorbed oxygen species at surface oxygen vacancy sites (Figure e). − The atomic percentage of surface oxygen vacancies increases from 54.7% to 80.6%, as the initial Ti content in TiO x EM increases from 0 to 60 wt % (Figure e and Figure S4b).…”

Anisotropy of the Electric Field Gradient in Two-Dimensional α-MoO3 Investigated by 57Mn(57Fe) Emission Mössbauer Spectroscopy

“…The physics behind the multiple above-mentioned applications is linked to the material phenomenology, which includes defects such as oxygen vacancies, point defects, and impurity doping, all of which can be studied by Mössbauer spectroscopy (MS), as shown in earlier studies [13][14][15].…”